The first combined sewer overflows since September 2016 started about 3:50 p.m. after 2.8 inches of rain had been measured at N. 36th St. and W. Fond du Lac Ave. and 2 inches of rain had fallen in Shorewood.

The deep tunnel had captured 307 million gallons of wastewater from combined sewer overflows, or 70% of its 432-million-gallon capacity, and flows were continuing to pour in when gates started closing and overflows were diverted to local waterways, MMSD Executive Director Kevin Shafer said.

The tunnel held 338 million gallons, or 78% of capacity, at 5:05 p.m.

Overflows of untreated wastewater to the rivers were started as an emergency measure to prevent sewage backups into residential and commercial businesses, Shafer said. No estimate of overflow volumes is available at this time.

"There are reports of sewer manholes popping off, leaving open holes in the street that lead directly into the sewer system," MMSD said on its "weather center" website. District officials urged motorists and pedestrians to use caution on city streets.

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Heavy rains caused flooding on N. 6th St. at the intersection with W. State St. in Milwaukee as thunderstorms rolled through the area.(Photo: Michael Sears / Milwaukee Journal Sentinel)

MMSD started a separate emergency measure at its Jones Island sewage treatment plant about 3:30 p.m. in an attempt to free space in the deep tunnel, officials said.

Operators at Jones Island started pumping wastewater from the deep tunnel directly to the plant's disinfection facility. The flow bypassed secondary treatment but disinfected with chlorine to kill bacteria before it was mixed with fully treated wastewater and discharged to the lake.

Jones Island has a capacity to treat wastewater at a rate of up to 300 million gallons a day. At 5:05 p.m. Monday, the plant was treating wastewater at a rate of 316 million gallons a day.

On Sept. 7 and 8, 2016, more than 109.2 million gallons of untreated wastewater poured into the Milwaukee, Menomonee and Kinnickinnic rivers during intense storms that dropped more than 3 inches of rain across the metropolitan area.